Sitter, Willem de (1872–1934)

De Sitter worked extensively on the motions of the satellites of Jupiter,
determining their masses and orbits from decades of observations. He redetermined
the fundamental constants of astronomy and determined the variation of the
rotation of the Earth. He also performed statistical studies of the distribution
and motions of stars, but today he is best known for his contributions to
cosmology. His 1917 solution to Einstein's field equations showed that a
near-empty universe would expand. Later, he co-authored a paper with Einstein
(1932) in which they argued that there might be large amounts of matter
which do not emit light, now commonly referred to as dark matter.